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The boundary layer plays a key role in several aspects of hurricane dynamics. Here we focus on its contribution to the balanced circulation. Previous studies, whilst including centrifugal terms, have not included an explicit balance in the boundary layer. Here, we improve the balanced theory to include an Ekman balance, the so-called frictional axisymmetric vortex (FAV). This approach is analogous to semigeostrophic theory that includes realistic boundary-layer diffusion: semigeotriptic (SGT) theory. We formulate the FAV for an axisymmetric system in cylindrical polar coordinates. We then derive a Sawyer-Eliassen equation for the vertical circulation. Example solutions for idealised hurricane-scale and synoptic-scale vortices are compared. [ABSTRACT FROM AUTHOR]

A recent meta‐study presented distributions of the possible impacts of climate change on the frequency and intensity of tropical cyclones for the six main tropical cyclone basins, summarizing many previous studies. However, it is not obvious how to interpret the distributions given in the meta‐study. I describe algorithms that can be used to interpret the changes in frequency and intensity in a mutually consistent way. I then investigate the nature of the relationships between the changes in frequency and intensity and find that the intensity changes are just a different way of looking at the information provided by the changes in frequency. The mean of the mean intensity distribution increases in all basins. In some basins this increase is associated mostly with an increasing frequency of the most intense tropical cyclones, while in other basins it is associated mostly with a decreasing frequency of less intense tropical cyclones. I consider the implications for a simple measure of risk that combines changes in frequency and intensity, and find that the most extreme risks increase for five out of six of the basins using this measure. The results of this study can be used to produce mutually consistent simulations of changes in tropical cyclone frequency and intensity. These can be used for further analysis of the effects of climate change on tropical cyclones. However, the results come with the major caveat that there are many uncertainties in the inputs and in the various steps of analysis. Finally, I give recommendations for which metrics could usefully be extracted from climate models in future studies of tropical cyclones under climate change to help with interpretation and impacts studies. [ABSTRACT FROM AUTHOR]

An idealized, three‐dimensional, numerical simulation of tropical cyclone evolution in a quiescent environment on an f‐plane is used to explore aspects of the cyclone's life cycle in the context of the rotating‐convection paradigm. In the 20‐day simulation, the vortex undergoes a life cycle including a gestation period culminating in genesis, a rapid intensification phase, a mature phase, a transient decay and re‐intensification phase, a second mature phase and a rapid decay phase. During much of the life cycle, the flow evolution is highly asymmetric, although important aspects of it can be understood within an azimuthally averaged framework, central to which are a boundary‐layer control mechanism and a new ventilation diagnostic. The boundary‐layer control mechanism provides an explanation for the gradual expansion of the inner core of the vortex. The ventilation diagnostic characterizes the ability of deep convection within a given radius to evacuate the mass of air ascending out of the boundary layer within that radius. The transient decay and re‐intensification phase is not associated with an eyewall replacement cycle, but rather with a hitherto undescribed process in which the eyewall becomes fragmented as a rainband complex forms beyond it. This process is interpreted as an interplay between the boundary layer and ventilation. The final rapid decay of the vortex results from the ever increasing difficulty of deep convection to ventilate the air exiting the boundary layer. Any unventilated air flows radially outwards in the lower troposphere and leads to spin‐down because of the approximate conservation of mean absolute angular momentum. If found in real cyclones, such transience or final decay might be erroneously attributed to ambient vertical wind shear. The results support the hypothesis that, even in a quiescent environment, isolated tropical cyclone vortices are intrinsically transient and never reach a globally steady state. [ABSTRACT FROM AUTHOR]

Data from an idealized three‐dimensional numerical simulation of tropical cyclone intensification are used to calculate Lagrangian air parcel trajectories emanating from the inflow layer that develops beneath the upper‐tropospheric outflow layer. It is found that about half of these trajectories end up in the outflow layer itself. The other half slowly subside to the mid‐ to upper troposphere, below the outflow layer, and drift slowly outwards as a result of a relatively weak overturning circulation in that region. Calculations show that pseudo‐equivalent potential temperature is not approximately conserved along the air parcel trajectories, indicating that the turbulent diffusion of heat and moisture and/or the latent heat changes by freezing or melting along the trajectories is appreciable in the mid‐ and upper troposphere. [ABSTRACT FROM AUTHOR]

Two methods for solving the Eliassen equation for the corresponding balanced secondary circulation of a numerically simulated, high‐resolution tropical cyclone vortex are compared. In idealized calculations for a symmetrically stable vortex, both methods (successive overrelaxation [SOR] and multigrid) converge and the solutions are broadly similar. In more typical cases, where the vortex has regions of inertial or symmetric instability, it is necessary to coarsen the data from the numerical simulation to determine the balanced secondary circulation. A convergent solution can be obtained with the multigrid method for a finer grid spacing than with the SOR method. However, the multigrid method fails to converge when the vertical grid spacing is similar to that of the numerical simulation. Results using both methods confirm the inability of the balance formulation to capture the strong inflow and resulting tangential wind spin‐up in the frictional boundary layer during a period of rapid intensification. Typical tropical cyclone simulations show an inflow layer just beneath the upper‐level outflow layer, and the corresponding balanced secondary circulation may show such an inflow layer also. However, caution is called for in attributing this inflow layer to a balanced flow response driven by the distribution of diabatic heating and tangential momentum forcing. Our study suggests that this inflow layer is likely an artifact of the ad hoc regularization procedure that is necessary to keep the Eliassen equation globally elliptic in regions of inertial and/or symmetric instability. [ABSTRACT FROM AUTHOR]

Four idealized, high‐resolution (500 m horizontal grid spacing), numerical simulations are used to investigate the evolution of convective structures during tropical cyclogenesis. The simulations all begin with a weak initial axisymmetric cloud‐free vortex in a quiescent environment, but differ in the moisture level of the initial sounding and whether or not ice microphysical processes are considered. Irrespective of experimental setup, there is only a short period where shallow or congestus clouds dominate. The shallow cloud phase is slightly extended with the drier initial environmental sounding. The composite structure of the convective elements sampled changes markedly throughout the genesis period. For much of the genesis phase, vertical profiles of the mean convective cell show significant amounts of anticyclonic vorticity produced in cells in the inner core. Towards the end of the genesis phase, there is a large increase in the production of cyclonic vertical vorticity in inner‐core convection, and cyclonic vorticity becomes dominant at low‐mid levels. The evolution from roughly equal strength vertical profiles of cyclonic/anticyclonic vorticity at low‐mid levels to profiles where cyclonic vorticity dominates occurs at relatively low system wind speeds (Vmax less than 10 m·s−1). This finding indicates a change in the structure of vortical convection prior to rapid intensification. In outer‐core convection, there are roughly equal strength vertical vorticity dipoles produced throughout the genesis period. [ABSTRACT FROM AUTHOR]

Three‐dimensional numerical simulations of tropical cyclone intensification with sufficient vertical resolution have shown the development of a layer of strong inflow just beneath the upper‐tropospheric outflow layer as well as, in some cases, a shallower layer of weaker inflow above the outflow layer. Here we provide an explanation for these inflow layers in the context of the prototype problem for tropical cyclone intensification, which considers the evolution of a vortex on an f‐plane in a quiescent environment, starting from an initially symmetric, moist, cloud‐free vortex over a warm ocean. We attribute the inflow layers to a subgradient radial force that exists through much of the upper troposphere beyond a certain radius. An alternative explanation that invokes classical axisymmetric balance theory is found to be problematic. We review evidence for the existence of such inflow layers in recent observations. Some effects of the inflow layers on the storm structure are discussed. [ABSTRACT FROM AUTHOR]

An idealized, three‐dimensional, 1 km horizontal grid spacing numerical simulation of a rapidly intensifying tropical cyclone is used to extend basic knowledge on the role of mean and eddy momentum transfer on the dynamics of the intensification process. Examination of terms in the tangential and radial velocity tendency equations provides an improved quantitative understanding of the dynamics of the spin‐up process within the inner‐core boundary layer and eyewall regions of the system‐scale vortex. Unbalanced and non‐axisymmetric processes are prominent features of the rapid spin‐up process. In particular, the wind asymmetries, associated in part with the asymmetric deep convection, make a substantive contribution (∼30%) to the maximum wind speed inside the radius of this maximum. The analysis provides a novel explanation for inflow jets sandwiching the upper‐tropospheric outflow layer which are frequently found in numerical model simulations. In addition, it provides an opportunity to assess the applicability of generalized Ekman balance during rapid vortex spin‐up. The maximum tangential wind occurs within and near the top of the frictional inflow layer and as much as 10 km inside the maximum gradient wind. Spin‐up in the friction layer is accompanied by supergradient winds that exceed the gradient wind by up to 20%. Overall, the results affirm prior work pointing to significant limitations of a purely axisymmetric balance description, for example, gradient balance/Ekman balance, when applied to a rapidly intensifying tropical cyclone. [ABSTRACT FROM AUTHOR]

We present idealized, three‐dimensional, convection‐permitting numerical experiments to evaluate the premise of the revised theory of tropical cyclone intensification proposed by Emanuel,. The premise is that small‐scale turbulence in the upper tropospheric outflow layer determines the thermal stratification of the outflow and, in turn, an amplification of the system‐scale tangential wind field above the boundary layer. The aim of our article is to test whether parametrized small‐scale turbulence in the outflow region of a developing storm is an essential process in the spin‐up of the maximum tangential winds. Compared with the control experiment, in which the small‐scale, shear‐stratified turbulence is parametrized in the usual way based on a Richardson number criterion, the vortex in a calculation without a parametrized representation of vertical mixing above the boundary layer has similar evolution of intensity. Richardson number near‐criticality is found mainly in the upper‐level outflow. However, the present solutions indicate that eddy processes in the eyewall play a significant role in determining the structure of moist entropy surfaces in the upper‐level outflow. In the three‐dimensional model, these eddy processes are largely realizations of asymmetric deep convection and are not obviously governed by any Richardson‐number‐based criterion. The experiments do not support the premise on which the new theory is based. The results would appear to have ramifications for recent studies that invoke the new theory. [ABSTRACT FROM AUTHOR]

Analyses of dropsonde data collected in Hurricane Edouard (2014) just after its mature stage are presented. These data have unprecedentedly high spatial resolution, based on 87 dropsondes released by the unmanned NASA Global Hawk from an altitude of 18 km during the Hurricane and Severe Storm Sentinel (HS3) field campaign. Attempts are made to relate the analyses of the data to theories of tropical cyclone structure and behaviour. The tangential wind and thermal fields show the classical structure of a warm‐core vortex, in this case with a secondary eyewall feature. Additionally, the equivalent potential temperature field (θe) shows the expected structure with a mid‐tropospheric minimum at outer radii and contours of θe flaring upwards and outwards at inner radii. With some imagination, these contours are roughly congruent to the surfaces of absolute angular momentum. However, details of the analysed radial velocity field are quite sensitive to the way in which the sonde data are partitioned to produce an azimuthal average. This sensitivity is compounded by an apparent limitation of the assumed steadiness of the storm over the period of data collection. Radius–height cross sections of selected fields derived from the dropsonde data: (a) tangential velocity component, contour interval 5 m/s, shading indicated on the side bar in m/s, and absolute angular momentum, black lines, contour interval 5 × 105 m2/s; (b) temperature perturbation, contour interval 2 K (positive values), 1 K (negative values), shading indicated on the side bar in K; (c) radial velocity component, contour interval 3 m/s, shading indicated on the side bar in m/s; (d) equivalent potential temperature, contour interval 10 K, shading indicated on the side bar in K, and absolute angular momentum, black lines, contours as in (a); (e) relative humidity, contour interval 10%, shading indicated on the side bar in %; and (f) a zoomed‐in version of (d) at heights below 3 km. [ABSTRACT FROM AUTHOR]

Many previous diagnoses of the global kinetic energy budget for a tropical cyclone have given prominence to the global integral of a pressure–work term in the generation of kinetic energy. However, in his erudite textbook Atmosphere–Ocean Dynamics, Adrian Gill derives a form of the kinetic energy equation in which there is no such explicit source term. In this article we revisit the interpretations of the generation of kinetic energy given previously in light of Gill's analysis and compare the various interpretations, which are non‐unique. Further, although global energetics provide a constraint on the flow evolution, in the context of the kinetic energy equation they conceal important aspects of energy generation and consumption, a finding which highlights the limitations of a global kinetic energy budget in revealing the underlying dynamics of tropical cyclones. The figure shows the radius–height structure of the azimuthally averaged pressure–work term identified by Anthes as the dominant generation term in the global kinetic energy equation, towards the end of a period of rapid intensification of an idealized tropical cyclone simulation. It shows several regions of energy generation and consumption which, owing to the fact that the averaged radial pressure gradient is everywhere positive, reflects the structure of the radial velocity. It is seen that the net global generation of kinetic energy is the residual after the cancellation of large positive and negative contributions. There is a large generation of kinetic energy in the frictional boundary layer and only a small contribution from the classical spin up mechanism. [ABSTRACT FROM AUTHOR]

The evolution of an idealized tropical‐cyclone‐like vortex forced by a prescribed distribution of diabatic heating rate and near‐surface frictional force is studied using a recently developed, prognostic, axisymmetric balance model. Starting with a prescribed tangential wind field, a series of calculations is carried out in which the strength of the diabatic heating rate is varied while keeping the formulation for the frictional force the same. When the strength of the heating rate falls below a certain value, the secondary circulation it generates is no longer able to oppose the tendency of the boundary layer to produce a layer of outflow in the lower troposphere, above the boundary layer. In these circumstances, this net outflow advects absolute momentum surfaces outwards above the boundary layer and the tangential velocity there declines. In the balance model, and presumably in reality, intensification of the tangential velocity requires the diabatic heating rate to be strong enough to ventilate the inflow in the frictional boundary layer, thereby averting frictionally induced outflow just above the boundary layer. In reality, the strength and location of diabatic heating will be strongly influenced by the boundary‐layer dynamics and thermodynamics, a coupling not present in the current model formulation. The evolution of an idealized tropical‐cyclone‐like vortex forced by a prescribed distribution of diabatic heating rate and near‐surface frictional force is studied using a recently developed prognostic axisymmetric balance model. When the strength of the heating rate falls below a certain value, the secondary circulation it generates is no longer able to oppose the tendency of the boundary layer to produce a layer of low‐level outflow in the lower troposphere, above the boundary layer. In these circumstances, the net outflow just above the boundary layer advects absolute momentum surfaces outwards, the tangential velocity there declines and the vortex weakens. [ABSTRACT FROM AUTHOR]

A recent idealized numerical study of tropical cyclogenesis and subsequent intensification using warm‐rain‐only microphysics is extended to examine the modifications brought about by a representation of ice processes. It is found that the time taken to reach cyclogenesis is more than twice that in the equivalent warm‐rain‐only simulation. The subtle reasons for the difference in the length of the gestation period are discussed. A mid‐level vortex forms during the early gestation period when ice processes are present, but not when warm‐rain‐only processes are present. Axisymmetric balance calculations show that the spin‐up of this mid‐level vortex is related to the different spatial distribution of diabatic heating rate in the presence of ice, which leads to a system‐scale radial influx of absolute vorticity in the middle troposphere. The tropical‐cyclone vortex that forms in the simulation with ice is similar to that in the warm‐rain‐only simulation, with the strengthening frictional boundary layer exerting a progressively important role in focusing inner‐core deep convection. This vortex develops in situ on a much smaller scale than the mid‐level vortex and there is no evidence that it is a result of the mid‐level vortex being somehow carried downwards, as has been suggested previously by some researchers. Some implications of the results in relation to previous theories of tropical cyclogenesis are discussed. [ABSTRACT FROM AUTHOR]

Three high-resolution numerical simulations are carried out to investigate the effects of initial vortex size on tropical cyclogenesis and intensification, starting with a relatively weak initial vortex with a maximum tangential wind speed of 5 m s−1, but located at different radii. In all simulations, there is a progressive organization of convectively induced, cyclonic relative vorticity into a monopole structure. As the initial vortex size is increased, the organization occurs later. In addition, the size of the vorticity monopole, the sizes of the inner and outer core tangential wind circulations and the lifetime intensity of the vortex all become larger. An explanation for this behaviour is offered in terms of a boundary-layer control of the overturning circulation. The findings are consistent with those of previous studies that bypass the genesis stage and start with appreciably stronger initial vortices. [ABSTRACT FROM AUTHOR]

The structure of an eyewall within an intense tropical cyclone is known to influence its intensity strongly. Furthermore, intense tropical cyclones often undergo an eyewall replacement cycle, a process in which an outer eyewall forms, contracts and replaces an inner eyewall. During this process, two concentric eyewalls will exist. To examine the effects of such structures, analytical solutions of the transverse circulation equation associated with a balanced vortex model have been proposed in the literature. For a single eyewall, the analytical solution is defined over three regions-the eye, eyewall and far-field-while in the case of concentric or double eyewalls one subdivides the domain into five regions-the eye, inner eyewall, moat, outer eyewall and far-field. The goal of the present article is to present a stability analysis for steady-state vortex solutions corresponding to tropical cyclones with either single or double eyewalls. We first obtain general steady-state solutions and a general approach for the stability or instability of such states involving a perturbation analysis (which makes use of small perturbations to the vortex structure in each of the three or five regions). We then make several simplifying assumptions on the form of the stationary solutions, so as to make the stability analysis tractable while maintaining physical relevance. The stability criteria obtained depend on the model parameters as well as on the structure of the perturbations undergone by the steady-state solutions. Making use of parameter values calibrated from the literature, we then provide bifurcation diagrams giving regions of stability or instability. Such analytical stability results for steady-state vortex solutions of the transverse circulation equation complement existing numerical stability results in the literature. [ABSTRACT FROM AUTHOR]

A recent idealized, high-resolution, numerical model simulation of tropical cyclogenesis is compared with a simulation in which the surface drag is set to zero. It is shown that, while spin-up occurs in both simulations, the vortex in the one without surface drag takes over twice as long to reach its intensification begin time. When surface friction is not included, the inner core size of the simulated vortex is considerably larger and the subsequent vortex intensity is significantly weaker than in the case with friction. In the absence of surface drag, the convection eventually develops without any systematic organization and lies often outside the radius of azimuthally averaged maximum tangential winds. The results underscore the crucial role of friction in organizing deep convection in the inner core of the nascent vortex and raise the possibility that the timing of tropical cyclogenesis in numerical models may have an important dependence on the boundary-layer parametrization scheme used in the model. [ABSTRACT FROM AUTHOR]

The hybrid Ensemble Kalman Filter - Variational (EnKF-Var) data assimilation (DA) system based on Grid-point Statistical Interpolation (GSI) is extended for the Hurricane-WRF model (HWRF). Background ensemble forecasts initialized by the EnKF are used to provide the flow-dependent error covariance to be ingested by GSI using the extended control variable method. The hybrid system is then applied to assimilate airborne radar data. In this article, the newly developed HWRF hybrid system capable of assimilating airborne radar observations is introduced. The impact of using variously estimated background error covariances on tropical cyclone (TC) core analyses and subsequent forecasts is explored by a detailed study of Hurricane Sandy (2012) and by systematic comparison of various sensitivity experiments for multiple cases during the 2012-2013 seasons. The hybrid system using the HWRF EnKF ensemble covariance (Hybrid-HENS) is able to correct both the wind and mass fields in a dynamically and thermodynamically coherent fashion. In contrast, the wind and pressure adjustments by GSI three-dimensional variation (GSI3DVar) using the static covariance are inconsistent. The wind and pressure relation in the covariances derived from the GFS ensemble (Hybrid-GENS) improves upon the static covariance, but is still inconsistent compared to that of HWRF. Verifications against independent flight-level and Stepped Frequency Microwave Radiometer (SFMR) wind data, and Hurricane Research Division (HRD) radar wind composite reveal that the Hybrid-HENS system improves the analysed TC structure upon both GSI3DVar and Hybrid-GENS. Hybrid-HENS and Hybrid-GENS improve the track, minimum sea-level pressure (MSLP) and Vmax forecast relative to GSI3DVar. Hybrid-HENS further improves track forecasts compared to Hybrid-GENS. Hybrid-HENS provides the largest positive impact of the airborne radar data. In comparison, GSI3DVar shows consistently negative impact of the data when analysing the structure and verifying track forecasts. Blending the static background error covariance in the hybrid system improves the maximum wind forecast while little benefit is found in the analysed structures and the MSLP and track forecasts. [ABSTRACT FROM AUTHOR]

Idealized high-resolution numerical simulations of tropical cyclogenesis are presented in a model that represents deep convection by a warm rain process only. Starting with an initially weak, cloud-free, axisymmetric warm-cored vortex (maximum wind speed 5 m s−1 at a radius of 100 km), rapid vortex intensification begins after a gestation period on the order of 2 days. From a three-dimensional perspective, the genesis process is similar to that in the rotating convection paradigm for vortex intensification starting with a much stronger initial vortex ( Vmax = 15 m s−1). The patterns of deep convection and convectively amplified cyclonic relative vorticity are far from axisymmetric during the genesis period. Moreover, the organization of the cyclonic relative vorticity into a monopole structure occurs at relatively low wind speeds, before the maximum local wind speed has increased appreciably. Barotropic processes are shown to play an important role in helping to consolidate a single-signed vorticity monopole within a few hours near the intensification begin time. The rotating convection paradigm appears adequate to explain the basic genesis process within the weak initial vortex, providing strong support for a hypothesis of Montgomery and Smith that the genesis process is not fundamentally different from that of vortex intensification. In particular, genesis does not require a 'trigger' and does not depend on the prior existence of a mid-level vortex. [ABSTRACT FROM AUTHOR]

Widely held arguments attributing the increasingly rapid intensification of tropical cyclones to the increasing 'efficiency' of diabatic heating in the cyclone's inner core region associated with deep convection are examined. The efficiency, in essence the amount of temperature warming compared with the amount of latent heat released, is argued to increase as the vortex strengthens on account of the strengthening inertial stability. Another aspect of the efficiency ideas concerns the location of the heating in relation to the radius of maximum tangential wind speed, with heating inside this radius seen to be more efficient in rapidly developing a warm-core thermal structure and, presumably, a rapid increase in the tangential wind. A more direct interpretation of the increased spin-up rate is offered when the diabatic heating is located inside the radius of maximum tangential wind speed. Further, we draw attention to the limitations of assuming a fixed diabatic heating rate as the vortex intensifies and offer reasons, on these grounds alone, as to why it is questionable to apply the efficiency argument to interpret the results of observations or numerical model simulations of tropical cyclones. Moreover, since the spin-up of the maximum tangential winds in a tropical cyclone takes place in the boundary layer and the spin-up of the eyewall is a result of the vertical advection of high angular momentum from the boundary layer, it is questionable also whether deductions about efficiency in theories that neglect the boundary-layer dynamics and thermodynamics are relevant to reality. [ABSTRACT FROM AUTHOR]

Differences in the vertical velocity characteristics associated with various cloud populations are evaluated for two simulated cases of tropical cyclone (TC) rapid intensification (RI) under varying wind shear. Within the radius of maximum wind (RMW), preceding RI in the low-shear TC (hurricane Ike of 2008) increased updraught magnitudes for the top 1% of the distribution at 7 km occur, while in the high-shear scenario (hurricane Earl of 2010) RI is led by increased updraught magnitudes of the top 1% of the distribution at 12 km. Three-dimensional analyses of individual updraughts relative to their peak altitude enables direct quantification of processes associated with shallow cumuli, cumulus congestus, deep convection failing to penetrate the tropopause, and convective bursts (CBs). Mean profiles for each convective regime reveal positive contributions in certain variables of each updraught variety towards RI with positive diabatic heating, absolute vorticity, and moisture convergence roles. Within the RMW, CBs are shown to be the primary diabatic heating and vertical mass and vapour flux contributors in both simulations, while Ike includes secondary contributions from deep convection and congestus in addition to noteworthy diabatic heating from apparent stratiform processes that act to spin up the mid-level vortex. Inner-core moisture convergence has divided contributions from shallow cumuli, cumulus congestus, and CBs. CBs act to enhance potential vorticity to the greatest amount and over the deepest vertical layer at low levels. The simulation results overwhelmingly support the aggregate importance of vertically developed deep convection and its associated ice processes to the initiation and maintenance of RI. [ABSTRACT FROM AUTHOR]

Two 1-km Weather Research and Forecasting model simulations of the rapid intensification (RI) periods of hurricanes Ike (2008) and Earl (2010), under low and high wind shear respectively, are performed to evaluate mechanisms linked to the initiation and maintenance of RI. Despite similar simulated intensification time series, the exhibited hydrometeor characteristics differ between the two cases since Ike possesses less asymmetry and less vigorous convection than Earl. An objective method to quantitatively estimate the radius of maximum wind (RMW) with respect to height is introduced for quantifying the primary circulation character of each storm, permitting analysis of where heating is most efficient for driving warm core growth and subsequent intensification. Following eye development in each, diabatic heating remains embedded within the RMW. Earl's RI onset coincides with an absolute maximum in diabatic heating occurring within the RMW, while Ike possesses relatively less diabatic heating inside the RMW throughout and lacks an absolute peak associated with RI. The majority of diabatic heating within the RMW of both cases occurs at subfreezing temperatures, indicative of the importance of clouds associated with ice processes in these RI simulations. Earl's diabatic heating peak is tied to a maximum in convective burst activity residing within the RMW. Earl's preference for more sparse but intense convection is associated with increased sea surface temperatures, surface heat fluxes, moisture convergence, and near-surface equivalent potential temperature relative to the broader, weaker convective character in Ike. [ABSTRACT FROM AUTHOR]

An improved version of a minimal model for a tropical cyclone is described. The model is used to revisit some fundamental aspects of vortex behaviour in the prototype problem for tropical cyclone intensification. After rapidly intensifying to a mature phase in which the maximum tangential wind speed remains quasi-steady for a few days, the vortex ultimately decays. In a 20-day simulation, the vortex never becomes globally steady. In particular, the upper anticyclone continues to expand for the duration of the integration. These results are consistent with those of recent studies using more sophisticated numerical models. As in the latter models, an important feature of the dynamics of spin-up is the development of supergradient winds in the boundary layer and the vertical advection of the associated high tangential-momentum air from the boundary layer to spin up the eyewall region. This mechanism, while consistent with some recently reported results, is not part of the classical theory of spin-up. [ABSTRACT FROM AUTHOR]

In this study, the multiple nested state-of-the-art Weather Research and Forecasting (WRF) model is used to investigate the impact of the subgrid-scale (SGS) vertical turbulent mixing parametrization on hurricane eyewall asymmetric structures and the formation of eyewall mesovortices. Hurricane Isabel (2003) was simulated by a series of numerical experiments with different SGS vertical turbulent mixing parametrizations including the Yonsei University, Mellor-Yamada-Janjic, Mellor-Yamada-Nakanishi-Niino 2.5 level and Mellor-Yamada-Nakanishi-Niino 3 level schemes. The simulations show that the vertical turbulent mixing scheme not only substantially affects the SGS vertical transport of heat and moisture but also has an important bearing on the storm axisymmetric structure, eyewall mesovortices and other resolved asymmetric features in the vicinity of the hurricane eyewall. The analyses show that the vertical turbulent mixing processes provide a mechanism to affect the barotropic instability for generation of eyewall mesovortices through changing the vortex basic state potential vorticity (PV) field and generating eyewall disturbances with different frequencies. Our numerical experiments show for given external conditions that the magnitude and vertical distribution of the eddy exchange coefficients are the key factors that regulate the characteristics of eyewall disturbances. Such a modulation of eyewall structure by the eddy exchange coefficients is realized through the complicated interaction among SGS vertical turbulent mixing, mesoscale structures, diabatic heating and barotropic instability. [ABSTRACT FROM AUTHOR]

Some conclusions from a recent numerical modeling study regarding the potential intensity of tropical cyclones are shown to be attributable to a relatively short model integration time. Three-dimensional simulations that are long enough to allow for a quasi-steady state of maximum intensity show a result consistent with theory, i.e., that maximum intensity is inversely proportional to surface drag coefficient. An alternative hypothesis for how tropical cyclones can intensify in numerical models in the absence of surface drag is also offered. [ABSTRACT FROM AUTHOR]